3DS:Safe Mode

Safe Mode (Mode sans échec), aussi appelé SAFE_FIRM, est un composant du firmware d'usine de la 3DS qui permet à l'appareil d'effectuer une mise à jour du système sans charger le menu HOME ni la plupart des fonctions du système. Pour accéder au Safe Mode, maintenez L + R + (D-Pad Up) + A puis démarrez. Si ça a fonctionné, la 3DS affichera l'écran de Mise à jour du système.
Il n'y a pas de combinaisons de touches alternatives, donc si l'un des boutons nécessaires ne fonctionne pas, vous ne pourrez pas accéder au Safe Mode.

Son utilité

Exploits

In 11.16.0, the recommended CFW method is unSAFE_MODE, which uses a flaw in Safe Mode's Internet Settings to install custom firmware. It uses a flaw that was already fixed in firmware 3.0, but Nintendo is more reluctant to update Safe Mode to patch exploits, since an unmodded 3DS would have no way to fix a brick by doing a system update if SAFE_FIRM were also bricked.

There are two other Safe Mode-based exploits that are now unusable:

safecerthax was an exploit for original 3DS and original 3DS XL models that exploited under Safe Mode the same SSLoth vulnerability available before 11.14.0 on NATIVE_FIRM. This was patched in 11.15 by patching SAFE_FIRM's SSLoth in the same way.
safehax allowed the console to load a .bin payload after being loaded from Homebrew Launcher due to SAFE_FIRM not being patched for firmlaunch-hax (while NATIVE_FIRM was patched since 9.5.0), and the console being able to load the Safe Mode firmware after the console was booted into normal firmware. This exploit was given a partial patch in version 11.3 and fully patched in version 11.4.

Redownloading system applications

Safe Mode reinstalls most deleted or corrupted system titles, like a regular System Update, which can fix some related issues.

Troubleshooting

If a crash on boot is caused by corrupted/deleted system titles or extremely outdated firmware, Safe Mode will stay unaffected as its titles are different from the ones used by NATIVE_FIRM.

As written above, it may repair the titles causing the brick, but if the problem is still not fixed, you will need GodMode9, so keep the custom firmware installed. If you cannot in any way access GodMode9, you will most likely need a flashcart compatible with ntrboot to fix the issue.